Let the Achievement Hunt Continue
Following the proven popularity and success of Microsoft 's Xbox achievements system, the Steam achievements system was introduced with the launch of The Orange Box late in 2007. Achievements became available with the recent update to Steamworks, but Half-Life 2: Episode Two, Portal and Team Fortress 2 were the first games to include them. Since then, the achievement system has grown in popularity and Steam now offers a wide selection of games featuring the achievements system.
Unlike Microsoft's achievements system, the Steam achievements have no associated point score attached to them. Valve, the developer behind both Steam and The Orange Box, have stated that the reason behind this is that achievements are available for mod-makers as well. It would be possible for a random mod-maker to create a mod that awards the players with a huge amount of points, which would render the whole system pointless. Literally. The first mod to include Steam achievement was D.I.P.R.I.P Warm Up.
Another reason why the achievements have no associated points, is that Valve sees achievements like a great way to enhance replayability by patching in more. Take for instance the popular class-based shooter Team Fortress 2; when it launched it featured 17 achievements for players to unlock. Now, many patches later, the game contains 518 unique achievements, which gives the users even more reasons to keep playing. Each class has its own set of achievements, encouraging switching between them.
The options the users have to get an overview of the rarity of their achievements:
Look up the stats manually for each game an http://steamcommunity.com/stats/, where it is possible to see how big a percentage of all users that have unlocked each achievement.
For detailed information about each game's achievement list, refer to the official site.